The invention relates to a transceiver which minimizes self jamming from reflected electromagnetic signals.
Radar system performance is degraded by electromagnetic signal reflection off a Casagrain antenna's subreflector. That reflection "self jams" the radar by increasing the receiver's noise floor. This noise floor limits the radar's ability to discriminate low-level reflections off distant objects from the higher intensity electromagnetic signals reflected from the subreflector. Minimizing that noise floor, therefore, improves the performance of the radar.
Past radar system designs sought to minimize "self jamming" signals from antenna reflections. One approach, a Frequency Modulated Continuous Wave radar system, matched the length, of transmission lines from the radar's transmitter and local oscillator to the front-end mixer. The mixer would then detect the arrival of the signals and thereby their phase noise. A voltage controlled oscillator 20 (FIG. 1) transmitted an electromagnetic signal to circulator 22. The circulator 22 allowed signal transmission only to the next downstream circuit component, the subreflector 24. Subreflector 24 reflected some electromagnetic signals to reflector 26 and some signals back to circulator 22. Electromagnetic signals reflected from reflector 26 ventured to a distant target. The target reflected some of those signals back to reflector 26. Reflector 26, in turn, reflected some of those captured signals to subreflector 24 which, in turn, reflected some of those signals to circulator 22. Circulator 22 directed signals, received from reflector 24, to mixer 28. Mixer 28 detected the frequency difference between the signal of voltage controlled oscillator 20 and the signal received from circulator 22. That frequency difference output from mixer 28 was proportional to the distance between the radar and the target. The mixer input from voltage controlled oscillator 20 traveled along a transmission line 29 having a length sufficient to generate a time delay in that mixer input. The time delay was equal to the round trip propagation time of the "self jamming" electromagnetic signal reflected from the subreflector 24 during signal transmissions. This time delay "matching" reduced the detection of phase noise by mixer 28, minimized the "self jamming" of the radar, and thereby allowed detection of more distant targets with lower signal transmission power.
An alternative approach to minimizing electromagnetic signal "self jamming" has been to alter the shape of subreflector 24. The purpose of the different shape was to produce an electromagnetic field "hole" 32 (FIG. 2) in the electromagnetic field of the main reflector 26 to minimize an energy path back to mixer 28.
The above described alternative designs still allow some amount of "self jamming" signal from the subreflector to contaminate the radar signal processing circuitry. The invention, rather than eliminating the "self jamming" signal reflections, uses them as the local oscillator for the mixer and eliminates the need for another local oscillator input to the mixer.